The present disclosure relates to techniques for taking accurate seismic measurements and, more particularly, to calibrating and eliminating noise in a seismic sensor by applying a specified amount of current to the moving coil of the seismic sensor.
The techniques presented herein relate to devices for sensing vibrations in earth formations such as electrodynamic sensing devices, including geophones and seismometers that have a moving coil placed in a magnetic field in a centered position. The present disclosure may be applicable to other types of vibration transducers, either in sensing or transmitting operation.
Seismic measurements and/or monitoring detect the vibrations in the earth resulting from a source of seismic energy that are sensed at discrete locations by sensors. In some applications, the output of the sensors is used to determine the structure of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic signals from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. For example, the sensed seismic signals may be direct signals that are derived from micro-seismicity induced by fracturing or reservoir collapse or alteration, or reflected signals that are derived from an artificial source of energy.
Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense particle motion arising from a seismic source.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface or on the wall of a borehole which penetrates the earth, moves in the direction of propagation of the energy (in the example of P-waves). If the axis of the geophone is aligned with the direction of motion, however, the moving coils mounted on springs inside the geophone stay in the same position causing relative motion of the coils with respect to the housing. When the coils move in the magnetic field, a voltage is induced in the coils which can be output as a signal. The response of a geophone is frequency dependent.
In order to ensure proper working operation of a geophone, typically the geophone is calibrated at the factory, periodically after manufacture and/or before each use. In this, geophone manufacturers and vendors generally do not perform any calibrations on their geophone units before the units are sold to customers. Rather the manufacturers provide assurances that the response of the geophone units are within specified tolerance ranges at a specific temperature such as room temperature. However, such tolerance guarantees are not a substitute for proper calibration of the geophone units. Consequently, many purchasers of geophone units perform their own calibration tests on the purchased geophone units before deploying such units in the field or during field use.
However, conventional geophone calibration tests are often inadequate for assuring the desired precision of the geophone measurements typically demanded for many of today's seismic measurement activities. In certain calibration techniques to measure the DC resistance (DCR) of the moving coil, current is injected into the coil and the resistance is determined from the voltage that appears across the coil. However, since the moving coil is also sensitive to vibrations of the geophone the DCR measurement accuracy depends on the environmental noise. If the geophone sensitivity is high, the measured noise is large and the accuracy of the DCR measurement is degraded. Furthermore, the inaccuracy of the DCR value influences other seismic sensor parameters, such as sensitivity and damping factor, since DCR is a basic value for calculating geophone parameters.
Accordingly, it will be appreciated that there exists a desire to improve upon conventional geophone calibration techniques in order to improve the accuracy of seismic measurements.
The limitations of conventional seismic sensors noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known sensor calibration techniques. The above should be sufficient, however, to demonstrate that seismic sensor techniques existing in the past will admit to worthwhile improvement.